The invention relates to a circuit arrangement for operating an exhaust-gas probe, especially an NOx probe.
Exhaust-gas probes of this kind are used in motor vehicles for determining the excess-air factor (lambda) and the NOx values. These exhaust-gas probes are, in most cases, operated so that the potentials of the electrodes are fixedly set in a controlled manner and the pump currents are measured. This can, for example, take place via so-called potentiostats which are conventional in electrochemistry. A typical control circuit of such a potentiostat is disclosed, for example, in the publication of A. J. Bard entitled xe2x80x9cElectrochemical Methodsxe2x80x9d (1980), FIGS. 13.4.6.
Control circuits of this kind include several operational amplifiers which all have different offsets. These different offsets of the operational amplifiers lead to errors in the measuring result of the exhaust-gas probes.
The offsets of the individual operational amplifiers add to a sum which is difficult to determine and which is also especially dependent upon temperature. To obtain a precise measuring result, a trimming of the individual operational amplifiers and their temperature characteristics is necessary.
It is an object of the invention to provide a circuit arrangement for operating an exhaust-gas probe which is so improved that the circuit arrangement makes possible the adjustment of the potentials having respective offsets which differ very little relative to each other.
Especially the above-mentioned matching or trimming of the operational amplifiers is to be avoided with the circuit arrangement of the invention.
The circuit arrangement of the invention is for operating an exhaust-gas probe including a NOx double chamber sensor. The exhaust-gas probe includes: a heatable solid-state electrolyte body having first and second pump chambers; diffusion barriers for separating the chambers from each other and from the exhaust gas; a third chamber communicating with the atmosphere; an external pump electrode exposed to the exhaust gas; a first oxygen pump electrode disposed in the first pump chamber; a second oxygen pump electrode disposed in at least one of the first and second pump chambers; a nitrogen oxide pump electrode disposed in the second pump chamber; an air reference electrode disposed in the third chamber; and, the circuit arrangement including: circuit means for applying pregivable voltages to the electrodes, respectively, and for generating, in a controlled manner, the following: a first oxygen pump current between the first oxygen pump electrode and the external pump electrode; a second oxygen pump current between the second oxygen pump electrode and the external pump electrode; and, a nitrogen oxide pump current between the nitrogen oxide pump electrode and the external pump electrode; and, the circuit means including: only one pump voltage generating circuit unit; switching means for switching the pump voltage generating circuit unit between respective ones of the pump electrodes; and, the pump voltage generating unit functioning to generate, in a controlled manner, all of the voltages applied to the pump electrodes in dependence upon respective reference voltages.
The pump voltage generating circuit unit can be switched over by a switching means between the individual pump electrodes and generates, in a controlled manner, all voltages applied to the pump electrodes in dependence upon the respective reference voltages. For this reason, all possibly occurring offsets of this pump voltage generating circuit unit are present to the same extent for all voltages applied to the pump electrodes and do not disturb the measuring result because only differences of the pump currents are measured in exhaust-gas sensors of this kind. The pump voltage generating circuit unit can be switched between the pump electrodes and therefore only one disturbing offset can occur. For this reason, no temperature compensation or trimming is needed in contrast to known circuits because of the circuit parts provided for each pump voltage. Furthermore, the generation of the pump voltage by only one single circuit unit is advantageous also with respect to the manufacture and assembly costs.
The pump voltage generating circuit unit advantageously has a single operational amplifier, which compares the reference voltages to the voltages applied to the pump electrodes and is switchable via the switching means and minimizes deviations of the voltages, which are applied to the pump electrodes, from the reference voltages. This operational amplifier is preferably a high precision balanced and temperature-compensated operational amplifier which has a minimal offset. A configuration of this kind of the pump voltage generating circuit unit is also especially cost effective. This is in addition to the technical advantages which are present in a small offset as well as an easy manipulation.
To minimize the deviation, the output of the operational amplifier is advantageously connected in each case via the switching means to three integrators assigned to the pump electrodes which integrate up the fault signals outputted by the operational amplifier. Three current measuring circuits are arranged downstream of the three integrators, respectively. The three current measuring circuits measure the pump currents flowing into the pump electrodes and output voltage values proportional thereto. In this way, three control loops are provided and possibly occurring offsets of the integrators and/or the current measuring circuits are insignificant for the measurement result to be obtained with the exhaust-gas probe. The only fault causing offset is generated by the pump voltage generating circuit unit in the form of an operational amplifier. This offset, however, occurs for all control loops in the same manner because the operational amplifier can be switched into the three control loops via the circuit means.
The circuit means is preferably configured in CMOS technology.
Operation is advantageously via a clock generator periodically with a frequency which preferably lies in the kilohertz range so that the operational amplifier is switched over in the kilohertz range into the three control circuits for generating the respective pump voltages at the individual electrodes.